1. Field of the Invention
The present invention relates to a device and a method for determining the starting capability of an internal combustion engine.
2. Detailed Description of Prior Art
Vehicles with a conventional internal combustion engine require a battery to stabilize the on-board power system voltage and to start the internal combustion engine. In the majority of all vehicles this is a lead acid battery, which can comprise a wide variety of embodiments.
During travel, this battery is charged by a generator driven by an internal combustion engine, and during the stationary state of the internal combustion engine the battery is discharged when the load is connected. If the battery is discharged too strongly during the stationary state, this can lead to a situation in which it is no longer possible to start the internal combustion engine.
Within the framework of reducing the CO2 emission, the internal combustion engine is to be completely switched off when the vehicle is stationary (for example at a traffic light) and the battery is to be charged by the generator, in particular in the overrun mode of the internal combustion engine. This strategy tends to increase the risk of a negative charge balance of the battery, which in extreme cases can cause stranding, that is to say an inability to start the internal combustion engine.
Compared to the diagnosis of stationary batteries (for example assisting the power supply in the event of a power failure, buffering power peaks during the generation of power) there are normally no catch points, that is to say no absolutely determinable battery characteristic values for the diagnosis (for example definitively full, definitively flat, definitively in complete rest etc.) in mobile applications (passenger cars, trucks, buses and also in fork lift trucks etc.). The state of the battery usually fluctuates between 70% and 95% in the state of charge, and the current is never precisely 0 A and a temperature range from −40° C. to 90° C. is possible, wherein the temperature is usually not constant during operation.
In order to diagnose a battery, it can be described, for example, by an electric model that requires as input variables the battery current, the battery voltage, and the battery temperature. There are also solutions in which the current or the voltage is not used directly for diagnosis of the battery but instead the battery resistance is used at various frequencies. The voltage is normally measured here by an analog/digital (A/D) converter connected downstream of a voltage divider. The analog variables of the current and the temperature are firstly each changed into a voltage value which is then converted in each case by an A/D converter into a numerical value. The detection of the current is carried out indirectly across the voltage drop on a shunt resistor or by a magnetic field which is induced by the current (for example Hall effect). For the measurement of the temperature, temperature-dependent resistance materials such as positive temperature coefficient (PCT) thermistors or negative temperature coefficient (NTC) thermistors are suitable.
The voltage is ideally measured at the poles of the battery. The entire battery current, i.e. the sum of the currents of all the loads, is ideally measured directly at one of the two poles, and the temperature is ideally measured in the interior of the battery. The battery current is partially estimated from the difference between the currents of the generator and the on-board power system and/or from the battery voltage.
However, alternatively, current, voltage and/or temperature can also be measured at other positions. The resulting error must then, however, be corrected/interpolated numerically by suitable algorithms. This is done, in particular, for the temperature, which is normally measured outside the battery. The A/D conversion of all three measured values is performed frequently (for example in an intelligent battery sensor) in the spatial vicinity of the battery. The A/D conversion can, however, also take place in any desired control unit, provided that the measurement signals are sufficiently strong to be able to be transmitted as far as the control unit without interference; for example under certain circumstances the signal can be boosted in a Hall sensor.
The measurement of the current has proven to be a cost driver. For this reason, in the past there have already been approaches involving diagnosing the battery without measuring the battery current. However, the objective was mostly to estimate the state of charge or a negative charge balance at which in total the battery is discharged to a greater extent than it is charged.
However, for the determination of the state of charge of a battery methods are also conceivable which are based on measuring the acid density (visual, acoustic, inductive etc.) The acid density correlates directly to the state of charge of the battery. However, these methods have previously not become widely accepted. An exception is what is referred to as the “magic eye”. The “magic eye” is embodied as a viewing window in the housing of the battery and indicates a measure of the acid content or the state of charge of the battery visually by way of color change.
However, the starting capability of the vehicle per se played only a subordinate role in this context since the driver always switched off the internal combustion engine and therefore was ultimately responsible for his own mobility. With the introduction of the start/stop system the focus of the battery diagnosis has shifted. The restarting of the vehicle at the traffic light which jumps to “green” is the core of the present discussion.